The two phrases 'fill in' and 'fill out' mean the same thing, but use opposite words. I am going to fill in this form. I am going to fill out this form. Same thing, different words. This alone is not so noteworthy; however, the difference in the phrase is in the the words 'out' and 'in'. Alone, they are opposites. How in the world did these two phrases come about?

Dictionary.com offers no clarification:

28. fill in, a. to supply missing or desired information: Fill in the facts of your business experience. b. to complete by adding detail, as a design or drawing: to fill in a sketch with shadow. c. to substitute for: to fill in for a colleague who is ill. d. to fill with some material: to fill in a crack with putty. e. Informal. to supply (someone) with information: Please fill me in on the morning news.

29. fill out, a. to complete (a document, list, etc.) by supplying missing or desired information. b. to become larger, fuller, or rounder, as the figure: The children have begun to fill out since I saw them last.

I can only imagine the frustration of someone trying to learn the nuances of English as a second language.

I realize though that they can mean different things in the case of substitution: 'The substitute teacher came to fill in for Mr. Smith.' - in this scenario, 'fill out' would probably be inappropriate. Why? They have the same meaning in other contexts, why should this application be any different? Maybe it's not and we're just not used to hearing it used that way: 'The substitute teacher came to fill out for Mr. Smith." This could be understood in the same way that someone 'fills out' their pants when they gain weight. In the gaps of Mr. Smith's absence, the substitute is 'filling out' the difference.

Gah, there are some things I may never understand. Also, I dislike Dictionary.com. I wish I could find a prescriptive dictionary online somewhere.